The invention relates to a method for materials processing by means of plasma-inducing high-energy radiation, especially laser radiation, in which the instantaneous intensity of the plasma radiation is measured at plural locations of a vapor capillary.
A method having the aforesaid features is known from DE 197 41 329 C1, wherein, instantaneous plasma intensities are measured parallel to an axis of an induced radiation at at least two measuring points. The measured plasma intensities are assigned to predetermined capillary geometry variables, i.e., for example, the depth of the vapor capillary, and control of the materials processing operation takes place as a function of these capillary geometry variables. This method utilizes a direct correlation between the observed plasma intensity and the formation of the vapor capillary to eliminate process errors by improved direct process monitoring. It has now been ascertained that this method is not applicable if the depth of the vapor capillary is comparable to its width. The analyzable relationship between plasma intensity and depth of penetration is no longer present.
By contrast, the object of the invention is to improve a method having the features cited in the introduction hereto in such a way that control of the materials processing operation can be influenced by direct process monitoring even when the depth of the plasma capillary is comparable to its width.
This object is accomplished in that shapes of two spaced-apart peak-intensity regions of the plasma radiation, or of another type of electromagnetic radiation emitted from the vapor capillary, and of a minimum region that can be formed between these two regions of extreme values are detected metrologically, in that metrologically detected shapes of the regions of extreme values are compared with predetermined region shapes, and in that control of the materials processing operation takes place as a function of deviations of the detected shapes from the predetermined region shapes.
It is important for the invention that exclusively areal analysis of the instantaneous plasma intensities of the detection region is performed. Another type of emitted electromagnetic radiation can be analyzed instead of the plasma radiation, for example emitted thermal radiation. Out of the metrologically detected region as a whole, shapes of predetermined regions are detected and analyzed. The predetermined regions are primarily peak-intensity regions. Such peak-intensity regions emerge laterally with respect to the axis of the laser beam. They are disposed roughly on a straight line perpendicular to the axis of the laser beam and identify a leading region at the edge of the vapor capillary and a trailing region at the edge of the same vapor capillary. A metrologically detectable minimum region may form between the two, depending on the conduct of the method. The shape of this minimum region can also be compared to a predetermined region shape and control of the materials processing operation can take place in accordance with the results of comparison of all the regions exhibiting extreme values. Since the method involves solely the analysis of planar shapes, it is independent of any metrologic detection of the depth of penetration of the vapor capillary, and is therefore especially well suited to thin workpieces. Materials processing by means of high-energy radiation can also be performed with on-line quality monitoring. This is possible especially in the welding of thin workpieces of unequal thickness, as in the case of so-called xe2x80x9ctailored blanks.xe2x80x9d In this case, the different thicknesses and coatings and the different properties of the materials necessitate special measures in the conduct of the method. A known characteristic feature in the welding of unequally thick workpieces is the lateral offset of the edges of the joint relative to the center of the weld seam. Even when the laser beam passes along the joint edge in an ideal manner, the weld seam forms with a lateral offset from the prepared joint edges. This offset must assume a set value. If the weld seam is situated farther into the thicker workpiece, the molten volume increases and the joint gap can be filled satisfactorily. If the weld seam is situated too far into the thick workpiece, the thin workpiece will not be melted adequately. If the weld seam is situated farther into the thinner workpiece, the overall molten volume decreases and the thicker workpiece is not melted through its entire thickness. Undesired undercuts are the result. The above-described undesirable method results can be controlled effectively by means of the previously described method steps, since differently fused workpieces are distinguishable by the different shapes of their regions of extreme values.
The method can be performed in such a way that control of the materials processing operation takes place when the shape of the minimum region deviates from a predetermined near-circular region shape. Such a method is important especially in cases where there is a minimum region that can be detected metrologically. Deviations from near-circular region shapes can cause welding defects and are, on the other hand, used to control the materials processing operation.
An improvement of the method that is designed to eliminate the above-described susceptibility to defects can be performed in such a way that control of the materials processing operation takes place when sharp regional boundaries are present in the regions of transition from the shape of the minimum region to the shapes of the peak regions. At the sharp boundaries between the shape of the minimum region and the brighter peak regions there is a joint edge that has not yet been melted by the laser beam. Thus, in a case where the values of the joint gap are not too great, for example one of the regions of extreme values may deviate from the predetermined region shape. For instance, a peak region is interrupted, i.e., darker in the region of a joint edge. The second peak region may then still be brighter, thereby indicating that the weldment joint is still present on the full width of the weld seam. In this case, the joint gap can still be spanned by means of the welding method. However, the observed deviations from the predetermined values do cause weld flaws, undercuts or even breakdown of the seam, in which too little molten material is present.
If the method is performed so that control of the materials processing operation takes place when the shape of a peak-intensity region that is in a leading position (in the feed direction) with respect to a workpiece that is being processed and is moving relative to the laser radiation and the shape of a trailing peak region deviate from predetermined region shapes. An improvement of the welding results can be achieved in this manner, in a case where the values of the joint gap are excessive, by the fact that not only is the leading peak-intensity region analyzed, but the trailing peak region is also analyzed simultaneously. By suitable control of the materials processing operation, the welding results are therefore also improved if both peak regions are interrupted by a minimum region along the joint edge.
Sufficient welding precision can also be achieved if control of the materials processing operation takes place when the deviation in shape exceeds a predetermined difference magnitude and/or a predetermined duration. Thus, the deviations must be of a set magnitude and must be detected for a set length of time. If they are not, no intervention is made in the welding process, so as to avoid burdening the control apparatus unnecessarily.
A further feasible way of developing the method can be to have the control of the materials processing operation take place as a function of angular positions assumed by a straight line passing through the peak-intensity regions with respect to a feed direction of the workpiece that is being processed and is moving relative to the laser radiation. Angular positions of a straight line can be detected with very little metrologic outlay and can be used for materials processing with little outlay for computer support.
A further feasible approach is to have the control of the materials processing operation take place when sporadically occurring, intensely radiating light spots are detected in a region of measurement that is detecting the shapes of the regions of extreme values by metrologic means. Such light spots indicate weld spatters that decrease the volume of the weld seam. The turbulence associated with such spatters can result in irregular weld beads.
The methods described hereinabove can be refined so that the control of materials processing in the case of workpieces having different thicknesses takes place when the minimum region deforms the peak region that is leading or trailing in the feed direction. Such deformation can occur in particular when the minimum region is well-defined owing to good root penetration. In this case, the deformation of the leading peak region signifies an undesirable deviation of the path of the laser beam into the thinner workpiece. Deformation of the trailing peak region, on the other hand, signifies an undesirable deviation of the path of the laser beam into the thicker workpiece. Both potentially undesirable deviations can be eliminated by control.
The method can also be performed in such a way that control of the materials processing operation takes place when two submaxima present on both sides of a joint path in a peak region that is in the leading position in the feed direction deviate from a predetermined symmetry. Symmetrical submaxima of the leading peak region occur particularly in the case of dummy welds, where there is consequently no joint gap between the joint edges and like materials are being processed. The presence of a joint gap, on the other hand, results in an asymmetry, which, however, can also be viewed as a predetermined symmetry in certain cases.
The method steps described hereinabove make it possible to obtain a good weldment from thin workpieces. Such a good weldment of sheet-stock workpieces or thin metal sheets is present when the lateral offset between the joint edges and the weld seam being produced has a set value, when the joint gap is completely filled with molten material and thus can be spanned connectingly, when at least one root penetrationxe2x80x94or morexe2x80x94is present, when the upper bead and the lower bead of the weld seam being produced have widths of set values, and when dimensional stability can be preserved throughout the welding process. These characteristics of a good weldment are to be reflected in quality and quantity in the relative positions and the shapes of the three intensity regions described hereinabove. Said characteristics are present to their full extent when each of the three regions has a predetermined shape, when the dark, third region, i.e. the minimum region, has a rounded shape, when the dark region is visible, when the bright, first region (upper bead) and the dark, third region (lower bead) have a width of a predetermined value, and when temporal and spatial deviations remain small. Concerning the visibility of the dark region, it should be noted that root penetration can, unfortunately, be present when the dark third region is not yet visible. Here, visibility of the third, dark region is only a necessary prerequisite for root penetration. Of course, root penetration is detected reliably when the dark, third region is visible.